To add to the list of @Un francophone I would recommend frengly.com - easy to use and it seems to work well enough for most things - providing that you can read enough to spot the typical machine translation faults.

Google Translate must be used very wisely, sometimes it translates good, other times completely wrong.
–
AlenannoAug 23 '11 at 10:27

1

@Alenanno : yes, but you can evaluate google translations and submit some better ones if you want.
–
Cédric JulienAug 23 '11 at 10:28

2

That's the main reason why it's not 100% reliable. Everybody can submit a different choice, so both experts and people who might make it wrong on purpose... :D You never know...
–
AlenannoAug 23 '11 at 10:29

For single words, I have had some good results with a Dutchonline dictionary - go figure. It does work way better if Dutch is involved, but English to French doesn't work that bad either, and sometimes you get definitions of the words too, which I think is a great help.

In any case, if by any chance you need Dutch <-> French, it'll be a good reference (you didn't specify source language in your question).

And, even if your question specifically says "online", I'll pretend I didn't see it and mention the good old-fashioned dictionary.

If you use the Google Chrome browser, it has translation built in. When you open a page that's primarily in any language other than your default, the browser detects it and puts a bar across the top of the page asking if you'd like the page translated. After translating, you can hover your mouse over any sentence to view the sentence in the original language.